Recently, substantial attention has been directed to the field of superconductors and to systems and methods for using such products. Substantial attention also has been to directed to systems and methods for providing a cold environment (e.g., 77 K or lower) within which superconductor products such as superconducting filter systems may function.
One device that has been widely used to produce a cold environment within which superconductor devices may function is the Stirling cycle refrigeration unit or Stirling cycle cryocooler. Such devices typically comprise a displacer unit and a compressor unit, wherein the two units are in fluid communication and are driven by one or more linear or rotary motors. Conventional displacer units generally have a “cold” end and a “hot” end, the warm end being in fluid communication with the compressor unit. Displacer units generally include a displacer having a regenerator mounted therein for displacing a fluid, such as helium, from one end, i.e., the cold end of the displacer unit, to the other end, i.e., the warm end, of the displacer unit. A piston assembly of the motor functions to apply additional pressure to the fluid when the fluid is located substantially within the warm end of the displacer unit, and to relieve pressure from the fluid when the fluid is located substantially within the cold end of the displacer unit. In typical cryocoolers, the piston and displacer units oscillate at 60 Hz. In this fashion, the cold end of the displacer unit may be maintained, for example, at 77 K, while the warm end of the displacer unit is maintained, for example, at 15 degrees above ambient temperature. Devices such as superconducting filters are then typically placed in thermal contact with the cold end of the displacer unit via a heat acceptor. Heat is transferred from the device to the heat acceptor. The heat transferred to the heat acceptor then passes to the helium gas contained in the displacer unit.
A typical motor used in a cryocooler comprises a piston assembly on which there is mounted a magnet ring assembly that transforms an oscillating magnetic energy field generated by motor coils to reciprocating mechanical energy that is applied to the piston assembly. For example, FIGS. 1 and 2 illustrate a prior art piston/magnet assembly 10, which includes a piston assembly 12 and a magnet ring assembly 14 mounted thereon. Referring specifically to FIGS. 3-6, the magnet ring assembly 14 includes eight magnets 16 that are cylindrically arranged to provide a radial magnetic field. To affix the magnets 16 in place, the magnet ring assembly 14 comprises an upper magnet holder 18, which includes an annular recess 20 that captures the tops 22 of the magnets 16, and a lower magnet holder 24, which includes an annular recess 26 that captures the bottoms 28 of the magnets 16. Preferably, the walls that straddle the annular recesses 20 and 26 are as thin as possible (e.g., 0.0050 inch), so that the thickness of the magnets 16 can be maximized. For purposes of structural integrity, the magnets 16 are held in place by bonding the tops 22 and bottoms 30 of the magnets 16 within the respective annular recesses 20 and 26. The magnet ring assembly 14 further comprises eight ring rods 32, which are located between the respective eight magnets 16 and TIG welded through corresponding holes 34 within the upper and lower magnet holders 18 and 24 to maintain the structural integrity of the magnet ring assembly 14.
Referring back to FIGS. 1 and 2, the piston assembly 12 comprises a cylinder 36 having a bore 38, a cylindrical piston 40 that axially moves within the bore 38 of the cylinder 36, a piston end cap 42 disposed mounted in the end of the piston 40, and a piston bracket 44 disposed on the opposite end of the piston 40. As best shown in FIGS. 1 and 4, the upper magnet holder 18 of the magnet ring assembly 14 comprises eight radially to circumferentially disposed mounting apertures 46, and the piston bracket 44 comprises eight corresponding circumferentially disposed mounting apertures 48, which are used to firmly bolt the magnet ring assembly 14 to the piston assembly 12, as illustrated in FIG. 1. So that the top surface of the upper magnet holder 18 is flush with the mounting surface of the piston bracket 44, the piston bracket 44 further includes eight radially disposed apertures 50 between the mounting apertures 48 to accommodate the ends of the ring rods 32 (shown best in FIG. 3) protruding through the upper magnet holder 18.
Referring still to FIG. 2, the piston assembly 12 further comprises gas bearings 52 that receive gas, e.g., helium, from a sealed cavity 54 within the piston 40. It should be noted that any suitable of gas bearings 52 can be used. In the illustrated embodiment, four circumferentially disposed pairs of gas bearings 52 (only two pairs shown) are used. A check valve 56 (best shown in FIG. 1) provides a unidirectional flow of gas from the front of the piston 40, through the sealed cavity 54 and out through the gas bearings 52. Preferably, the gas bearings 52 comprise orifices that are on the order of a one or two mils (e.g., 1.5 mils), so that only a small amount of gas escapes from the sealed cavity 54 though the gas bearings 52, thereby preserving the pressure that has built up in the sealed cavity 54 until the next stroke of the piston 40. Typically, only 2-5 percent of gas that is displaced by the piston 40 enters the sealed cavity 54 through the check valve 56.
Because the smallest drill bit currently is around 2.9 mils with a maximum length of about 30 mils, the orifices of the gas bearings 52 cannot be drilled. Instead, each of the gas bearings 52 includes an aperture 58 in which there is disposed a gas bearing restrictor in the form of a screw 60 that can be turned to adjust the rate of gas that flows through the gas bearing 52. That is, the length of the passage created by the threaded helix between the screw 60 and the aperture 58 can be decreased or increased by carefully rotating the screw 60 in and out of the aperture 58 until the correct flow rates are attained in all gas bearings 52. Alternatively, sapphire/ruby or glass orifices (not shown) with very small diameters can be used as the gas bearing restrictor to provide a consistent gas flow at the designed rate without requiring adjustment. These orifices, however, can only be made so long, and as will be described in more detail below, have reliability problems. The piston assembly 12 further comprises centering ports 62 (shown in FIG. 1), which provide a return gas circuit from region adjacent the back of the piston 40 to the region adjacent the front of the piston 40.
Due to the tight tolerances (typically, about 5 mils) between the magnet ring assembly 14 and adjacent laminations (only internal lamination 28 shown) that are disposed on both the inside and outside surface of the magnet ring assembly 14, the circularity of the magnet ring assembly 14 must be perfect or near-perfect, so that it does not rub against the adjacent laminations. For the same reason, the concentricity between the piston 40 and the magnet ring assembly 14 must be perfect or near-perfect. In addition, the magnets 16 must be in a perfect or near-perfect cylindrical equidistant arrangement, so that the generated magnetic field is radially uniform. In this manner, a uniform load will be provided to the gas bearings 52, thereby maximizing the efficiency of the piston assembly 12. Thus, it can be appreciated that great care must be taken when assembling the magnet ring assembly 14, resulting in often tedious and time consuming process that is magnified by the relatively large number of parts (eighteen-eight magnets, eight ring rods, two magnet holders) that make up the magnet ring assembly 14. Notably, magnet segments cannot currently be made as a single fully cylindrical piece due to magnetic technology limitations. Thus, multiple magnets must be painstakingly mounted within the upper and lower magnet holders 18 and 24. Also, the measures taken to ensure that the magnet ring assembly 14 and piston 40 are concentric along their lengths, namely, the drilling of the apertures 50 within the piston bracket 44 that accommodate the protruding ring rods 32, provide additional time-consuming steps. Furthermore, because the walls adjacent the annular recesses 20 and 26 of the respective upper and lower magnet holders 18 and 24 are preferably very thin, so that the thickness of the magnets 16 can be maximized, these walls are often inadvertently perforated, resulting in the scrapping of the respective magnet holder.
In addition, all eight screws 60 within the apertures 58 of the gas bearings 52 have to be iteratively adjusted and the flow rate measured throughout the fabrication process of the cryocooler to ensure that the gas bearings 52 exhibit the designed flow rate at the end of the final assembly process. Great care must be taken when rotating the screws 60 within the apertures 58, so that the heads of the screws 60 are not stripped. Occasionally, however, this will occur, requiring that the expensive piston assembly 12 be scrapped.
Reliability of the cryocooler is another concern. In the field of commercial Radio Frequency (RF) communications, it is desired that Stirling cycle cryocoolers provide maintenance free operation for tens of thousands of hours, and more preferably, at least forty thousand hours. After mere thousands of operational hours, however, cryocoolers that incorporated piston/magnet assemblies similar to the assembly 10 described above were failing. It was discovered that, when the piston 40 banged against the cylinder 36, the epoxy joints between the magnets 16 and the upper and lower magnet holders 18 and 24 would break and/or the magnet ring assembly 14 would go out of round, causing the magnet ring assembly 14 to rub against the adjacent laminations and/or unequal loading of the gas bearings 52. As a result, the magnet ring assembly 14 would deteriorate rapidly. Thus, the high energy transmitted to the magnet ring assembly 14 due to the high frequency application of the motor stresses the importance of the attachment technique between magnet and the magnet holder. It was also discovered that when sapphire/ruby or glass orifices are alternatively used as the gas restrictors, a static charge would build up as the gas flows through them at 60 Hz. As a result, very fine particles would collect within the very small diameters (typically about 0.0012 inch in diameter) and eventually plug them.
Thus, there is a need for an improved magnet ring assembly and gas bearing restrictor that can be used with piston assemblies, such as those found in cryocoolers.